I really need to add the info which branch is used somewhere in the about panel. Did you use the master or develop branch? the master is heavily outdated and the develop branch is almost a new program.

There is a floating tile called ImagePool. Add it somewhere in HISE (eg. the custom workspace). It contains a list of all loaded images. Right click on the one you want to reload and choose Reload file (it has a few other nice features like drag and drop, preview etc).

There are other pool tables for all other cached data types (audio files, samplemaps).